Chemical bonding between atoms

The electrons are mobile or delocalized. They move from one metal atom to another.

The electrons are transferred from metals to non-metals.

The transfer creates oppositely charged particles called ions.

The electrons are shared between the non-metals.

A covalent bond is created by the sharing of two electrons between atoms. This is represented by a dash between two atoms. eg. H-H

Nature of chemical bond

The attraction of positive metallic ions and the negative mobile valence electrons.

The attraction between a positive ion (cation) and negative ion (anion).

The attraction between the pair of negative electrons within the covalent bond and the adjoining positive nuclei

Examples

Gold Au, silver Ag and copper Cu

Sodium chloride NaCl and magnesium oxide MgO

Water H2O and methane CH4

*An ionic bond is defined when the electronegative values between the two atoms is greater than 1.7

Chemical bonding between molecules

Molecules are discrete particles that exist as single units. The atoms within a molecule are joined together by strong covalent bonds. The molecules are joined together in the solid and liquid states by weak intermolecular forces.

Intermolecular bonding

Intermolecular bonding is the attraction between molecules. The size of the intermolecular bond helps determine the physical state of the molecule. Gases have very weak intermolecular bonds.

The following image shows the intermolecular attraction between water molecules.

The dotted lines represent weak intermolecular bonds. The delta δ signs show the partial charges between the water molecules. The type of intermolecular bond that exists between water molecules is called a hydrogen bond.

There are three types of intermolecular bonds that exist between molecules

Dispersion forces

Dipole-dipole attraction

Hydrogen bonding

Description

Dispersion forces exist between all molecules.

Dispersion forces are due to the momentary attraction between the electron cloud of one molecule and the positive nuclei of an adjoining molecule.

A temporary dipole in one molecule is created by the random movement of the electron cloud. This induces a temporary dipole in another molecule. The molecules then become attracted to one another.

Dispersion forces are the weakest type of intermolecular bond. They do however, increase with molecular size. This is shown by the progressive increase in the boiling points of the alkane series of hydrocarbons.

Differences between the electronegativity values of the atoms in a covalent bond create a polar covalent bond.

In a polar covalent bond the electrons are unevenly shared.

If the polar bonds do not cancel one another out a permanent dipole (two poles, + -) is created in the molecule.

The attraction between the oppositely charged poles of one molecule with another molecule is called dipole-dipole attraction.

Hydrogen iodide, HI has two permanent poles due to the even sharing of electrons.

The electrons spend more time around the iodine atom.

Hydrogen bonding is a particular strong form of dipole-dipole attraction between molecules.

A large difference in the electronegative values of hydrogen with either nitrogen, oxygen or fluorine produces a very large dipole(+ -). ie. H-N, H-O or H-F

The dotted lines represent the hydrogen bonds between molecules of hydrogen fluoride, HF

Hydrogen bonding is the strongest type of intermolecular bond.

Examples

Hydrocarbons

Methane, CH4

Hexane, C6H14

Octane, C8H18

Hydrogen chloride, HCl

Hydrogen bromide, HBr

Hydrogen iodide, HI

Water, H2O

Ammonia, NH3

Hydrogen fluoride, HF

Ethanol, C2H5OH

Acetic acid, CH3COOH

Polar and non-polar molecules

Molecular covalent substances can be divided into two groups, polar molecules and non-polar molecules.

Molecule

Polar molecules

Non-polar molecules

Description

A polar covalent bond is created by the differences between the electronegativity values of the two bonded atoms. There is an unequal sharing of electrons in the covalent bond.

A molecule is polar if the overall sum of the polar bonds creates two permanent poles (+ -)

Positive or negative delta signs, δ+ or δ- are used to indicate the polar nature of molecule. Delta means 'partial charge'

Water, H2O contains two polar O-H bonds at 104.5° to one another. The molecule is polar in nature. Oxygen is δ- and each hydrogen δ+

A non-polar covalent bond is created by small differences between the values of the two bonded atoms. There is an equal sharing of electrons in the covalent bond.

A molecule is non-polar if it only contains non-polar covalent bonds or if it contains polar bonds which cancel out each other due to their position or symmetry. eg. Carbon dioxide CO2 is a linear molecule, O=C=O and boron trifluoride, BF3 is symmetrical.

Boron trifluoride is non-polar because the polar bonds cancel one another out due to their symmetry. The net polarity of the molecule is zero. No permanent poles are created.